Transmission unit and two-way signal conversion method

ABSTRACT

A transmission unit which transports digital signals between different network systems, effectively converting the signals to resolve their differences in the specifications. A first signal interface transmits and receives first network signals. A second signal interface transmits and receives second network signals. A downward converter produces lower-level signals by converting received first and second network signals to a lower hierarchical level at which the first and second network systems are compatible with each other in terms of logical signal structure. An upward converter produces a higher-level signal by converting each given lower-level signal to an upper hierarchical level which complies with the first or second network system. A loopback unit provides loopback paths to route the lower-level signals from the downward converter to the upward converter, so that the first and second network signal will be converted in both directions.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a transmission unit and a two-waysignal conversion method. More particularly, the present inventionrelates to a transmission unit which transports digital signals, and toa method to convert signal formats, transmission rates, hierarchicallevels, and other attributes in both directions.

2. Description of the Related Art

Synchronous Digital Hierarchy (SDH) and Synchronous Optical Network(SONET) specifications have been standardized and implemented as today'score data multiplexing technologies which provide an efficient way ofintegrating a wide variety of high-speed and low-speed services. Digitalsignals that this type of synchronous networks carry are organized inaccordance with prescribed hierarchical multiplexing levels, wherelower-order group signals (i.e., slower signals) are combined into ahigher-order signal (i.e., faster signal). Each network element acceptssuch higher-order multiplexed signals form the upstream element andretransmits them the downstream element, while extracting and insertingsome lower-order signals. Some nodes with cross-connect capabilitiesprovide consolidation and segregation of transport signals.

As an approach for improved maintainability of the above transportsystems, a signal loopback technique is disclosed in the JapaneseUnexamined Patent Publication No. 10-243008 (1998). According to thedisclosed technique, the signal in any specified time slot of alow-order group channel is looped back to another low-order groupchannel. This conventional technique, however, is limited to low-ordergroup signals of the same hierarchical series of multiplexing levels,and it is therefore not possible to loop back a signal from one channelto a different type of channel, such as from a high-order SONET channelto a high-order SDH channel. That is, conventional systems do notsupport looping back between networks having different signalspecifications. For operations and maintenance of varioustelecommunication services on a global scale, it is necessary to developan improved transport system which supports efficiently converting, notonly between SONET and SDH, but other signals having differentspecifications to one another.

SUMMARY OF THE INVENTION

Taking the above into consideration, an object of the present inventionis to provide a transmission unit which transmits and receives signalsbetween network systems, effectively converting signals to resolve thedifference in their specifications.

It is another object of the present invention to provide a two-waysignal conversion method which converts network-specific transportsignals effectively.

To accomplish the first object stated above, according to the presentinvention, there is provided a transmission unit which transmits andreceives digital signals over a first and second network systems. Thistransmission unit comprises a first signal interface, a second signalinterface, and a two-way signal converter. The first signal interfacetransmits and receives first network signals, while the second signalinterface transmits and receives the second network signals. The two-waysignal converter converts between the first network signals and thesecond network signals. To this end, the two-way signal convertercomprises a downward converter, an upward converter, and a loopbackunit. The downward converter converts the received first and secondnetwork signals down to a lower hierarchical level at which the firstand second network systems are compatible with each other in terms oflogical signal structure, thereby producing lower-level signals. Theupward converter, on the other hand, converts each given lower-levelsignal up to a higher hierarchical level which complies with the firstor second network system, thereby producing higher-level signals. Thelooping back unit interconnects the downward and upward converters atthe lower hierarchical level. That is, it loops back the producedlower-level signals to the upward converter, thereby causing thereceived first and second network signals to be converted into a secondand first outgoing network signals, respectively. These first and secondoutgoing network signals are sent out to the first and second networksystems through the first and second interfaces, respectively.

To accomplish the second object, according to the present invention,there is provided a two-way signal conversion method which convertsnetwork signals between a first and second network systems. This methodcomprises the following three steps. At step (a), lower-level signalsare produced by converting a first and second incoming network signalsdown to a certain lower hierarchical level at which the first and secondnetwork systems are compatible with each other in terms of logicalsignal structure. At step (b), higher-level signals are produced byconverting each given lower-level signal up to a higher hierarchicallevel which complies with the first or second network system. At step(c), loopback paths are provided, so that the lower-level signalsproduced at step (a) will be subjected to the step (b). This enables thelower-level signal resulting from the first incoming network signal tobe converted into an outgoing signal to the second network system. Also,the lower-level signal resulting from the second incoming network signalis converted into an outgoing signal to the first network system.

The above and other objects, features and advantages of the presentinvention will become apparent from the following description when takenin conjunction with the accompanying drawings which illustrate preferredembodiments of the present invention by way of example.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a conceptual view of a transmission unit according to thepresent invention;

FIG. 2 shows the operation of the proposed transmission unit;

FIG. 3 is a block diagram of a transmission unit;

FIGS. 4 to 6 show the location of stuff columns;

FIG. 7 depicts a conversion from VT-1.5 to TU-12;

FIG. 8 depicts a conversion from a high-order signal in which VC-4signals are multiplexed to another high-order signal in which VC-4-4csignals are multiplexed;

FIG. 9 depicts a conversion from M23 to M13 (or C-bit);

FIG. 10 depicts a conversion between transport signals including ATMcells;

FIG. 11 depicts a conversion from ATM-150 to ATM-600;

FIG. 12 depicts a conversion from “IP over WDM” to “IP over SONET”;

FIG. 13 is a block diagram of another transmission unit;

FIG. 14 is a block diagram of a two-way signal converter, which includesan AU pointer handler;

FIGS. 15 to 17 show how to distinguish between AU-3 and AU-4 signals;

FIG. 18 is a block diagram of another two-way signal converter, whichincludes an E1 byte handler;

FIG. 19 shows the location of E1 byte in an overhead;

FIG. 20 shows the details of E1 byte usage;

FIG. 21 is a block diagram of a transport system;

FIG. 22 is a block diagram of another transport system;

FIG. 23 is a flowchart of a two-way signal conversion method accordingto the present invention;

FIG. 24 is a diagram which shows a first system configuration accordingto the present invention;

FIG. 25 is a diagram which shows a second system configuration;

FIG. 26 is a diagram which shows a third system configuration;

FIG. 27 is a diagram which shows a fourth system configuration; and

FIG. 28 is a diagram which shows a fifth system configuration.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the present invention will be described belowwith reference to the accompanying drawings.

FIG. 1 is a conceptual view of a transmission unit according to thepresent invention. This transmission unit 1 receives a signal SA of afirst network system and converts it into a signal SB of a secondnetwork system. The transmission unit 1 also receives a signal SB of thefirst network system and converts it into a signal SA of the firstnetwork system. Those transport signals SA and SB are structured inmultiple levels according to their respective protocol specifications,as indicated by the nested rectangles in FIG. 1. As a result, they arepartly incompatible with each other in terms of higher-level structure,as in the case of SONET and SDH. The transmission unit 1 converts thosesignals in both directions simultaneously, although FIG. 1 only shows aone-way conversion, from the first network signal SA to the secondnetwork signal SB, for the sake of simplicity.

The transmission unit 1 comprises a first signal interface 2, a secondsignal interface 3, and a two-way signal converter 10. The first signalinterface 2 transmits and receives first network signals SA. The two-waysignal converter 10 comprises a downward converter 11 and an upwardconverter 12 and a loopback unit 13. The transmission unit 1 actuallyemploys multiple sets of these converter elements 11 and 12, as will beexplained later in FIG. 2.

The downward converter 11 produces a lower-level signal Cd by convertingthe received first network signal SA to a lower hierarchical level atwhich the first and second network systems are compatible with eachother in terms of logical signal structure. The upward converter 12, onthe other hand, produces a higher-level signal Cu by converting thelower-level signal Cd to an upper hierarchical level which complies withthe second network signal specifications. The loopback unit 13 providesa loopback path of the down-converted signal Cd from the downwardconverter 11 to the upward converter 12, so that the first networksignal SA will be converted into a signal compatible with the secondnetwork system. The second signal interface 3 transmits and receivessecond network signals SB. FIG. 1 shows that the second signal interface3 aligns the higher-level signal Cu into an outgoing second networksignal SB.

It should be noted again that the transmission unit 1 has another set ofdownward converter 11 and upward converter 12 to convert transportsignals in the other direction, from the second network signal SB to thefirst network signal SA. The additional downward converter 11, in thiscase is coupled to the second signal interface 3, and the additionalupward converter 12 is linked to the first signal interface 2. Theloopback unit 13 provides a path from the downward converter 11 to theupward converter 12 so as to loop back a lower-level signal Cd derivingfrom a received second network signal SB.

The transmission unit 1 operates with a network management console 20.This network management console 20 is used for operations andmaintenance of the transmission unit 1, including configurationmanagement for the above-described signal conversion functions.

Referring next to FIG. 2, the operation of the transmission unit 1 willbe briefly described. The illustrated transmission unit 1 is configuredto convert a SONET OC-192 signal with the transmission rate of 9.953280Gbps to an SDH Synchronous Transfer Module Level 64 (STM-64) signal withthe same rate. The transmission unit 1 also converts a SONET OC-48signal with the rate of 2.488320 Gbps to an SDH STM-16 signal with thesame rate. The two-way signal converter 10 thus comprises two downwardconverters 11-1 and 11-2 and two upward converters 12-1 and 12-2 toprocess two different signals concurrently. The first signal interface 2receives an OC-192 signal SA1 and supplies it to one downward converter11-1. The first signal interface 2 also receives an OC-48 signal SA2 andsupplies it to the other downward converter 11-2.

The downward converter 11-1 converts the received signal SA1 down to theTributary Unit Group-2 (TUG-2) level (6.912 Mbps) or lower. At thisTUG-2 level, the SONET and SDH specifications are compatible with eachother, allowing data to be passed from the OC-192 channel to the STM-64channel. The downward converter 11-2, on the other hand, converts thereceived signal SA2 down to TUG-2 level (6.912 Mbps) or lower, at whichthe data can be passed from the OC-48 channel to the STM-16 channel. Inthis way, the two downward converters 11-1 and 11-2 produce lower-levelsignals Cd1 and Cd2 at TUG-2 level.

The loopback unit 13 provides loopback paths to route one TUG-2 levelsignal Cd1 from the downward converter 11-1 to the upward converter 12-1for conversion to STM-64 rate, as well as to feed the other TUG-2 signalCd2 from the downward converter 11-2 to the upward converter 12-2 forconversion to STM-16 rate.

The upward converter 12-1 converts the given TUG-2 signal Cd1 up toSTM-64 level, thus producing an upper level signal Cu1. Likewise, theother upward converter 12-2 converts the other TUG-2 signal Cd2 up toSTM-16 level, thus producing another upper level signal Cu2. The secondsignal interface 3 sends out those upper level signals Cu1 and Cu2 ashigher-level SDH signals SB1 (STM-64) and SB2 (STM-16), respectively.

Although the above section has discussed the conversion from SONET toSDH, it should be noted that the transmission unit 1 has another set ofdownward and upward converters to provide an SDH-to-SONET bridge. Inthis case, the downward converter breaks down a higher-level SDH signalinto lower-level signals, and the upward converter multiplexes givenlower-level signals into a higher-level SONET signal.

The above explanation has assumed that the first network signals SA arefed directly from the first signal interface 2 to the downward converter11, and the upper level signals Cu are fed directly from the upwardconverter 12 to the second signal interface 3. As will be describedlater in FIG. 3, those signals may be converted to an adequate rate thatis suitable for the internal operations and processing in thetransmission unit 1.

Further, while the above-described transmission unit 1 converts betweenhigh-order group signals, the invention should not be limited to anyspecific group of signals, but can handle low-order group signals aswell. The first signal interface 2 may receive lower order SONETsignals, such as OC-3 (155.2 Mbps) or STS-12, and subject them toappropriate internal functions in the transmission unit 1 to yield STM-1or STM-4 signals. The resultant SDH signals are then output through thesecond signal interface 3.

Referring next to a block diagram of FIG. 3, another type oftransmission unit which processes transport signals after making themadapt to its internal interface specifications. Illustrated in FIG. 3 isa transmission unit la which converts AU-3 signals to AU-4 signals, orvise versa. Administrative Unit Level 3 (AU-3) is a signal form commonlyused in SONET transport systems, while AU-4 is often seen in SDHtransport systems. Note that, in the following explanation, the terms“mapping” and “demapping” will be used to refer to what have beendescribed so far as the upward conversion and downward conversion,respectively. Accordingly, the upward and downward converters will becalled “mappers” and “demappers.”

The illustrated transmission unit 1 a converts a SONET OC-192 signal SA1to an SDH STM-64 signal SB1 as follows. The first signal interface 2receives an OC-192 signal SA1 (9.953280 Gbps) and converts it intointernal signals D1 with a certain bit rate, which is, for example,STS-12 signals with the rate of 622.080 Mbps. In this way, thetransmission unit 1 a processes a high-rate transport signal afterdown-converting them to lower rate signals.

Unlike the transmission unit 1 described earlier in FIG. 2, thetransmission unit 1 a comprises a switch 4. With its signal pathselecting function, this switch 4 directs the internal signals D1 to anAU-3 demapper 11-3 which converts D1 down to the TUG-2 level or lower.

The loopback unit 13 provides a loopback path from the demapper 11-3 toa mapper 12-3. The mapper 12-3 makes a mapping from TUG-2 level to AU-4level, thereby producing remapped internal signals D1 a. The signals D1a are STM-4 signals with the rate of 622.080 Mbps. The switch 4 directsthe remapped internal signals D1 a to the second signal interface 3. Thesecond signal interface 3 multiplexes the remapped internal signals D1 aup to the STM-64 level and outputs the resultant signal as an STM-64signal SB1.

The illustrated transmission unit 1 a also converts an SDH STM-16 signalSB2 to a SONET OC-48 signal SA2 as follows. The second signal interface3 receives an STM-16 signal SB2 and converts it into internal signals D2with the STM-4 rate of 622.080 Mbps. With its signal path selectingfunction, the switch 4 directs the internal signals D2 to an AU-4demapper 11-4. The demapper 11-4 further breaks down the internalsignals D2 to TUG-2 level or lower. The loopback unit 13 provides aloopback path from the demapper 11-4 to a mapper 12-4. The mapper 12-4makes a mapping from TUG-2 level to AU-3 level, thereby producingremapped internal signals D2 a. These signals D2 a are STS-12 signalswith the rate of 622.080 Mbps. The switch 4 directs the remappedinternal signals D2 a to the first signal interface 2. The first signalinterface 2 multiplexes the remapped internal signals D2 a to OC-48level and outputs the resultant signal as an OC-48 signal SA2.

Referring again to FIG. 1, the proposed transmission unit 1 terminatesand adds a frame overhead when converting transport signals. Morespecifically, thedemapper (or downward converter) 11 in the integraltwo-way signal converter 10 terminates the overhead information of eachreceived frame when demapping SONET or SDH signals. Such informationincludes: Section Overhead (SOH), Path Overhead (POH), AU pointer, andTributary Unit (TU) pointer. The resultant lower-level signals arelooped back to an appropriate mapper (or upward converter) 12 throughthe loopback unit 13. During it upward conversion process, the mapper 12adds an SOH and POH to those signals, thereby raising them to an upperhierarchical level.

The function of terminating and adding overhead information need notnecessarily be concentrated in the two-way signal converter 10. Rather,this function may be implemented in the switch 4, first signal interface2, and second signal interface 3 in a distributed manner. For example,the following scenario would be possible when converting first networksignals into second network signals: The first signal interface 2terminates SOH, the switch 4 removes pointers, and then the demapper 11terminates POH. After that, the signal is looped back to the mapper 12,where POH is added. The switch 4 then inserts pointer information, andthe second signal interface 3 adds SOH.

To handle VC-3 signals, the locations of fixed stuff, the bytes carryingno overhead or payload, should be considered. FIGS. 4 to 6 show suchfixed stuff locations of VC-3 and other related frames. The location ofstuff columns within a frame depends on which hierarchical series ofmultiplexing levels the frame belong to, as described below.

According to the standard SDH multiplexing hierarchy, a SynchronousTransfer Module Level-1 (STM-1) signal carries a single AU-4 frame. AnAU-4 frame contains a single VC-4 frame in which three VC-3 frames aremultiplexed. The SDH standards also define another type of STM-1 signalwhich is composed of three AU-3 frames each containing a single VC-3frame. That is, VC-3 appears at two nodes in the hierarchical tree ofstandard SDH multiplexing levels. FIG. 4 shows such VC-3 frames as thelower-level components of AU-4, and FIG. 5 depicts a VC-4 framecontaining VC-3 frames of this type. FIG. 6, on the other hand, shows aVC-3 frame as the component of AU-3.

The STM-1 payload is 261 columns in length, regardless of whether it isorganized by a single AU-4 frame (i.e., single VC-4 frame) or three AU-3frames (i.e., three VC-3 frames). In the both cases, 252 columns out of261 columns are used to carry the user information at the intendedbitrate, and the remaining six columns are used for other purposes orfilled with stuff bytes. More specifically, in the case of AU-3 (FIG.6), two columns #30 and #59 of each of three VC-3 frames are defined asfixed stuff, making six stuff columns in total. In the case of AU-4(FIGS. 4 and 5), the six remaining columns are assigned to VC-4 POH, twofixed stuff columns #2 and #3 in VC-4 frame, and three TU-3 pointers(one for each VC-3 frame).

As seen from the above, the VC-3 frame format for AU-4 is not compatiblewith that for AU-3 in terms of the arrangement of stuff columns. This iswhy the two-way signal converter 10 has to consider the stuff columnlocations when converting transport signals. Take an AU-4 to AU-3conversion, for example. The demapper 11 converts a given AU-4 signaldown to a particular hierarchical level at which no stuff bytes areseen, the resulting signal being looped back to the mapper 12. Themapper 12 inserts an appropriate number of stuff bytes (i.e., stuffcolumns #30 and #59 of each VC-3 frame) to the given signal, thusmapping it to a higher-level signal.

Besides processing transport signals with the same rate as describedabove, the proposed two-way signal converter 10 can also convertssignals with different rates. More specifically, FIG. 7 shows aconversion from SONET Virtual Tributaries 2(VT-1.5) signals (1.5 Mbps)to SDH TU-12 signals (2 Mbps). The primary rate interface service inSONET networks operates typically at 1.5 Mbps. In SDH networks, on theother hand, the rate of 2 Mbps is commonly used. Each single transportsignal at VC-3 or equivalent level is capable of carrying up totwenty-eight VT-1.5 channels, or up to twenty-one TU-12 channels.

Referring to FIG. 7, the demapper 11 makes a demapping of 1.5 Mbps dataout of the incoming SONET VT-1.5 signal to yield a SDH VC-11 signal.This VC-11 signal is looped back to the mapper 12, which converts thesignal up to the TU-12 level operating at 2 Mbps. In this way, VT-1.5signals can be translated into TU-12 signals. From the above discussion,it would be appreciated that the same method applies to other similarrate conversions, such as TU-11 to TU-12 and TU-11 to VT-2.

The proposed two-way signal converter 10 may also be configured totranslate one signal to a different signal level within the samehierarchical series, such as from a high-rate signal carrying VC-4virtual containers to that conveying VC-4-4c concatenated containers.FIG. 8 depicts this type of conversion. The first signal interface 2receives an incoming signal SA3 with a transmission rate of 40 Gbps, 10Gbps, or 2.5 Gbps on which VC-4 virtual containers are multiplexed. Itconverts the received signal into internal signals D1 with a certainbitrate, e.g., STM-16 or STM-4 rate. With its signal path selectingfunction, the switch 4 directs the internal signals D1 to the demapper11. The demapper 11 demaps the received internal signals D1 down to theVC-4 level or lower. The loopback unit 13 provides a loopback path fromthe demapper 11 to the mapper 12. The mapper 12 maps the given VC-4signals to the VC-4-4c level, thereby producing remapped internalsignals D1 a with the STM-16 or STM-4 rate. The switch 4 routes theremapped internal signals D1 a to the second signal interface 3. Thesecond signal interface 3 converts those signals D1 a to VC-4-4cconcatenated channel signals with a rate of 40, 10, or 2.5 Gbps. In thisway, the first network signals SA3 is converted to the second networksignals SB3. It would be appreciated that the same method applies to theconversion in the opposite direction, as well as to other similarconversions, such as from VC-4-16c multiplexed signals to VC-3multiplexed signals.

According to the present invention, the two-way signal converter 10 mayalso be configured to convert Plesiochronous Digital Hierarchy (PDH)frames. PDH systems provide various types of frames at each multiplexinglevel. Digital Signal Level-3 (DS3) lines operating at 44.736 Mbps, forexample, carry M13, M23, and C-bit frames. Likewise, 1.544-Mbps DS1lines convey SF, ESF, and SLC-96 frames, and 34.368-Mbps E3 lines carrythe frames defined in the ITU-T recommendations G.751 and G.832. Thepresent invention enables these different PDH frames to be converted toeach other.

Illustrated in FIG. 9 is a conversion from M23 frames to M13 (or C-bit)frames at the DS3 level. In this conversion, the two-way signalconverter 10 demaps given M13 frames down to a certain signal level atwhich DS3 framing bits have been removed. The mapper 12 receives aloopback signal from the demapper 11 and maps it into an M13 (or C-bit)signal. The reverse conversion from M13 (or C-bit) to M23 can beperformed in the same way.

According to the present invention, the two-way signal converter 10 alsosupports transport signals containing ATM cells. FIG. 10 depicts aconversion between signals including ATM cells. More specifically, FIG.10 shows how “ATM over SONET” signals are converted into “ATM over SDH”signals. In the illustrated process, the demapper 11 demaps ATM cellsextracted from the received “ATM over SONET” signal. The mapper 12produces an “ATM over SDH” signal by inserting the ATM cells looped backfrom the demapper 11 thereto. In a similar way, the reverse conversionfrom “ATM over SDH” to “ATM over SONET” can be performed.

According to the present invention, the two-way signal converter 10 canalso be applied to the conversion of various Asynchrous TransferMode(ATM) interface specifications described in the ATM Forum standardsincluding ATM-25, ATM-50, ATM-150, and ATM-600. Referring to FIG. 11, aconversion from ATM-150 to ATM-600 is illustrated. The demapper 11demaps an incoming ATM-150 signal down to a certain level, e.g., ATM-25.The mapper 12 receives the resultant ATM-25 signals from the demapper 11and maps them into ATM-600 signals. In a similar way, the reverseconversion from ATM-600 to ATM-150 can be performed.

According to the present invention, the proposed two-way signalconverter 10 further supports the signal conversion between differenttransmission media used to transport Internet Protocal(IP) packets. Suchmedia include: “IP over WDM,” “IP over SONET,” “IP over SDH,” “IP overPPP,” “IP over ATM,” “IP over Ethernet.” The present invention enablesIP datagrams to be passed over those different media. Referring to FIG.12, a signal conversion from, for example, “IP over WDM” to “IP overSONET” is explained. The demapper 11 demaps the “IP over WDM” signalsdown to a certain level at which the “IP over WDM” and “IP over SONET”networks share a common format. The mapper 12 then produces “IP overSONET” signals by mapping thereto the common format signal that islooped back from the demapper 11. In a similar way, the reverseconversion from “IP over SONET” to “IP over WDM” can be performed.

According to the present invention, the two-way signal converter 10 maybe implemented as part of a lower-order group signal interface. FIG. 13is a block diagram of a transmission unit 100 which serves as a networkelement in an ATM network. This transmission unit 100 comprises a firsthigh-order group interface 2 a, a second high-order group interface 3 a,a switch 4, and a low-order group interface 101. Some of these elementshave already been explained earlier in FIG. 3.

The first and second high-order group interfaces 2 a and 3 a transmitand receive network signals at higher levels in the multiplexinghierarchy, as in the first and second signal interfaces 2 and 3discussed in FIG. 3. The low-order group interface 101 comprises a DS3interface 102 and an E3 interface 103, in addition to the two-way signalconverter 10 described so far. The DS3 interface 102 handles signals of“ATM over DS3,” while the E3 interface 103 handles signals of “ATM overE3.”

Consider, for example, a case where the “ATM over SONET” signals aresubjected to the conversion to “ATM over SDH” signals. In this case, thefirst high-order group interface 2 a receives a signal SA4 from the “ATMover SONET” network and converts it into an internal signal D1. With itssignal path selecting function, the switch 4 directs the internal signalD1 to an AU-3 demapper 11-5. The demapper 11-5 extracts ATM cells fromthe internal signal D1, thus demapping the received high-order signal.The loopback unit 13 provides a loopback path from the demapper 11-5 tothe mapper 12-5, which executes an AU-4 level mapping by inserting theATM cells. The resultant signal is referred to as a remapped internalsignal D1 a. The switch 4 routes the remapped internal signal D1 a tothe second high-order group interface 3 a. The second high-order groupinterface 3 converts the remapped internal signal D1 a to a secondnetwork signal SB1 for the “ATM over SDH” transport.

In parallel to the above operation, the DS3 interface 102 may receive apart of the demapped low-order signal from the demapper 11-5 and outputit to the “ATM over DS3” network, applying an appropriate signaltranslation. It is also possible for some signals received from the “ATMover DS3” network to be routed to the mapper 12-6 through the DS3interface 102. The mapper 12-6 converts those signals to the AU-3 levelSONET signal for the “ATM over SONET” network. The same applies to the“ATM over E3” signal interface.

While the above explanation of FIG. 13 has assumed an ATM-basedconfiguration of the low-order group interface 101, the presentinvention should not be limited to this particular type of transportsystem. The low-order group signals may include any PDH frames (e.g.,DS3, E3, DS1, E1) and IP packets for LANs.

While the embodiments of the invention have been explained so far underthe assumption that the switch 4 determines to which mapper or demappereach AU-3/AU-4 signal should be directed, the proposed two-way signalconverter may have a dedicated function to identify the AU pointer ofeach frame to determine the destination of those AU-3/AU-4 signals. Inthe following section, this functional block will be referred to as anAU pointer handler.

FIG. 14 is a block diagram of a two-way signal converter 10-1 whichincludes an AU pointer handler 14. The AU pointer handler 14 receives aninternal signal D1. Based on the AU pointer found in the receivedinternal signal D1, the AU pointer handler 14 determines whether theinternal signal D1 is of AU-3 series or AU-4 series (the terms “AU-3series” and “AU-4 series” refer to the two different hierarchical seriesof multiplexing levels explained earlier in FIGS. 4 to 6). The result ofthis test is then sent to either one of the demappers 11-5 and 11-6.After the mapping is finished, the AU pointer handler 14 inserts anappropriate AU pointer to the remapped signal, depending on which mapperis supplying the signal D1 a, the AU-4 mapper 12-5 or the AU-3 mapper12-6.

FIGS. 15 to 17 show how to distinguish between AU-3 and AU-4 signals.More specifically, FIGS. 15 and 16 show the H1 and H2 bytes in atransmission signal frame at the multiplexing level of STM-1 (SDH) orSTS-3 (SONET), which are called the AU pointer bytes. Since a singleSTM-1 (or STS-3) is capable of carrying three VC-3 frames, the overheadprovides three sets of H1 and H2 bytes. When the values contained inthose three sets (#1, #2, #3) are independent of each other, as shown inFIG. 15, those bytes are interpreted as three AU-3 pointers. When onlythe first set (#1) of H1/H2 bytes contain a pointer value and theremaining sets (#2 and #3) are dependent to the first one, as shown inFIG. 16, those bytes are interpreted as an AU-4 pointer. In the case ofAU-4 pointers, the second and third H1 bytes (Y) hold a value of“1001SS11” in binary notation, and the second and third H2 bytes are setto all ones (1*).

Referring now to FIG. 17, the AU pointers in an STM-4 frames isdepicted. Since four STM-1 frames are multiplexed into one STM-4 frame,the STM-4 AU pointers include four STM-1 pointers as indicated by thesuffixes #1 to #4 in FIG. 17. Accordingly, the STM-4 frames aredemultiplexed to the STM-1 level, and each STM-1 pointer is thenextracted and subjected to the AU-3/AU-4 test that has been described inFIGS. 15 and 16.

Another way to determine the destination of AU-3/AU-4 signals is toexamine the value of E1 byte contained in each section overhead. Toimplement this function, the proposed two-way signal converter 10 has anE1 byte handler.

FIG. 18 shows the structure of a two-way signal converter 10-2 includingan E1 byte handler 15. This E1 byte handler 15 receives an internalsignal D1 and examines its E1 byte to determine whether the internalsignal D1 is of AU-3 series or AU-4 series. The result of this test isthen sent to either one of the demappers 11-5 and 11-6. After themapping is finished, the E1 byte handler 15 sets an appropriate value toE1 byte of the remapped signal D1 a, depending on which mapper issupplying the signal D1 a, the AU-4 mapper 12-5 or the AU-3 mapper 12-6.

FIGS. 19 and 20 show the location and definition of E1 byte in theoverhead. More specifically, FIG. 19 depicts an STM-4 (SDH) or STS-12(SONET) overhead, where E1 byte is located on the first column of thesecond block. Referring to FIG. 20, the first two bits #1 and #2 of E1byte are unused. Bit #3, when it is set to “1,” indicates AU-4-16c (SDH)or STS-48c (SONET). Likewise, bit #4 indicates AU-4-4c or STS-12c; Bit#5 indicates AU-4#4 or STS-3c#4; Bit #6 indicates AU-4#3 or STS-3c#3;Bit #7 indicates AU-4#2 or STS-3c#2; Bit #8 indicates AU-4#1 orSTS-3c#1.

When bit #3 is set to zero, it means the frame is neither AU-4-16c norSTS-48c. When bit #4 is set to zero, it means the frame is neitherAU-4-4c nor STS-12c. When bits #5 to #8 are all set to zero, it meansthat the frame is AU-3 (SDH) or STS-1 (SONET).

Referring next to FIG. 21, a transport system according to the presentinvention is shown. This transport system 1-2 is a variation of thetransmission unit 1 described earlier in FIG. 1, where its originalfunctional blocks are implemented into two separate pieces of networkequipment, a first transmission unit 200 and a second transmission unit210.

The first transmission unit 200 comprises a first signal interface 201a, a first demapper 202 a, and a first mapper 203 a. The first signalinterface 201 a transmits and receives first network signals. The firstdemapper 202 a produces first lower-level signals Cd20 by converting agiven signal down to a lower hierarchical level at which the first andsecond network systems are compatible with each other in terms oflogical signal structure. The first mapper 203 a, on the other hand,produces a first higher-level signal Cu20 by converting given secondlower-level signals Cd21, which are supplied from the secondtransmission unit 210, to an upper hierarchical level which complieswith the first network signal specifications. The first higher-levelsignal Cu20 is sent out to the first network through the first signalinterface 201 a.

The second transmission unit 210 comprises a second signal interface 211a, a second demapper 212 a, and a second mapper 213 a. The second signalinterface 211 a transmits and receives second network signals. Thesecond demapper 212 a produces second lower-level signals Cd21 byconverting the received second network signal down to a lowerhierarchical level at which the first and second network systems arecompatible with each other in terms of logical signal structure. Thesecond mapper 213 a produces a second higher-level signal Cu21 byconverting the first lower-level signals Cd20, which are supplied fromthe first transmission unit 200, to an upper hierarchical level whichcomplies with the second network signal specifications. The second upperlevel signal Cu21 is sent out to the second network through the secondsignal interface 211 a.

FIG. 22 is a block diagram of another transport system 1-3. Thistransport system 1-3 is a divided version of the transmission unit ladescribed earlier in FIG. 3, where its original functional blocks areimplemented into two separate pieces of equipment, a first transmissionunit 300 and a second transmission unit 310.

The first transmission unit 300 comprises a first signal interface 301a, a first switch 302 a, a first demapper 303 a, and a first mapper 304a. The first signal interface 301 a receives a first network signal andconverts it into a first internal signal D30 with a certain fixed bitrate. It also outputs a first network signal which is produced byconverting a second remapped internal signal D31 a up to the requiredlevel. The first switch 302 a controls where to direct the firstinternal signal D30 and second remapped internal signal D31 a.

The first demapper 303 a produces first lower-level signals Cd30 byconverting the first internal signal D30 down to a lower hierarchicallevel at which the first and second network systems are compatible witheach other in terms of logical signal structure. The first mapper 304 areceives second lower-level signals Cd31 from the second transmissionunit 310 and converts them to an upper hierarchical level, therebyproducing a second remapped internal signal D31 a that is compatiblewith the first network specifications.

The second transmission unit 310 comprises a second signal interface 311a, a second switch 312 a, a second demapper 313 a, and a second mapper314 a. The second signal interface 311 a receives a second networksignal and converts it into a second internal signal D31 with a certainfixed bit rate. It also outputs a second network signal which isproduced by converting the first remapped internal signal D30 a up tothe required level. The second switch 312 a controls where to direct thesecond internal signal D31 and first remapped internal signal D30 a. Thesecond demapper 313 a produces second lower level signals Cd31 byconverting the given second internal signal D31 down to a lowerhierarchical level at which the first and second network systems arecompatible with each other in terms of logical signal structure. Thesecond mapper 314 a receives the first lower level signals Cd30 from thefirst transmission unit 300 and converts them to an upper hierarchicallevel, thereby producing the first remapped internal signal D30 a thatis compatible with the second network specifications.

According to the present invention, a two-way signal conversion methodis provided to convert transport signals between a first and secondnetworks. FIG. 23 is a flowchart of the proposed method, which comprisesthe following steps:

(S1) Producing lower-level signals by converting a first and secondincoming network signals down to a lower hierarchical level at which thefirst and second network systems are compatible with each other in termsof logical signal structure.

(S2) Looping back the produced lower-level signals at the lowerhierarchical level, so as to subject them to intended upward conversions(which will be provided in the next step).

(S3) Producing higher-level signals by converting each given lower-levelsignal up to a higher hierarchical level which complies with the firstor second network system, whereby the lower-level signal resulting fromthe first incoming network signal will be converted into an outgoingsignal to the second network system, and the lower-level signalresulting from the second incoming network signal will be converted intoan outgoing signal to the first network system.

According to one aspect of the proposed method, overhead informationcontained in the first and second incoming network signals is terminatedduring the downward conversion process. After loopback, overheadinformation is inserted to outgoing signals during the upward conversionprocess.

According to another aspect of the proposed method, stuff data containedin the first and second incoming network signals is located and removedduring the downward conversion. During the subsequent upward conversion,stuff data is inserted to appropriate part of the outgoing signals, soas to comply with the first and second network systems.

According to yet another aspect of the proposed method, the transmissionrates of network signals are converted from one to the other. Theproposed method supports various patterns of rate conversions, such as:TU-11 to TU-12, TU-11 to VT-2, VT-1.5 to TU-12, and VT-1.5 to VT-2.

According to still another aspect of the proposed method, ATM cells areextracted from the first or second incoming network signal during thedownward conversion process, and the ATM cells are inserted to theoutgoing signals during the subsequent upward conversion process.

According to still another aspect of the proposed method, networksignals containing IP packets are subjected to the conversions. Thefirst and second incoming network signals are converted down to signalshaving a common format, which are then upward-converted to higher-levelsignals.

According to still another aspect of the proposed method, various typesof network signal conversions are supported. One such type is two-wayconversions between high-order group signals belonging to differenthierarchical series of signals. Another type is two-way conversionsbetween low-order group signals belonging to different hierarchicalseries of signals. Still another type is two-way conversions between ahigh-order group signal and a low-order group signal which belong todifferent hierarchical series of signals.

According to still another aspect of the proposed method, various typesof transport systems can be converted. One such type is two-wayconversions between SDH signals and SONET signals. Another type istwo-way conversions between such signals that belong to the samehierarchical series of signals, but have different levels in thehierarchy. PDH signals and ATM signals can also be handled.

According to still another aspect of the proposed method, the first andsecond incoming network signals are converted to lower-level signalsaccording to their respective AU pointer types. It is also possible touse E1 byte or any other bytes in the overhead field of internalsignals, unless they are assigned for other purposes.

According to still another aspect of the proposed method, a networkmanagement console may be used for operations and maintenance of theconversions of the network signals.

The next section will present several possible system configurationswhere the proposed two-way signal converter is implemented as part of alow-order group interface. FIG. 24 shows a first such system, whichcomprises: a plurality of higher-rate interfaces 402 and 403, across-connect unit 404, and a plurality of low-order group interfaces410. The higher-rate interfaces 402 and 403 handle both AU-3 and AU-4signals. The cross-connect unit 404 acts as what has been described asthe switch 4. The low-order group interfaces 410 support DS1 (T1) and E1(D12) signals, providing a capacity of 28 channels per interface for DS1and 21 channels per interface for E1. The higher-rate interfaces 402 and403, on the other hand, support multi-channel transport signals ofSTM-64 (OC-192), STM-16 (OC-48), STM-4 (OC-12), or STM-1 (OC-3).

Whatever rates the transport signals may have, the internal main signalcircuits interconnecting the higher-rate interfaces 402 and 403,cross-connect unit 404, and low-order group interfaces 410 operate at aunified rate of 622 Mbps, which is equivalent to the STM-4 (STS-12)rate. Actually, this internal rate depends on the system configuration;a higher rate (e.g., 2.4 Gbps) or a lower rate (e.g., 155 Mbps) may beselected if it is more appropriate. The high-rate interfaces 402 and 403have demultiplexing and multiplexing functions to convert the networktransport signals to this internal signal level. Likewise, the low-ordergroup interfaces 410 have like functions to convert the low-order groupsignals (DS1, E1) to that signal level.

More specifically, the low-order group interfaces 410 are equipped withtwo types of demappers (demultiplexers/demappers) and mappers(mappers/multiplexers) for the conversion of 622 Mbps frames to/from DS1and E1 signals. One type is intended for AU-4 signals, and the othertype is for AU-3 signals; they are therefore referred to as “AU-4mappers and demappers” and “AU-3 mappers and demappers,” respectively.

As seen from FIG. 24, the low-order group interfaces 410 receive622-Mbps signals from the cross-connect unit 404 and demultiplex them tothe TUG-2 level. These TUG-2 signals are looped back and multiplexed tothe STM-n level (n: 1, 4, 16, 64 . . . ), which is an SDH signalequivalent to STS-3*n or OC-3*n in SONET. An STM-n signal containingAU-3 frames can be converted into another type of STM-n signalcontaining AU-4 frames, through the use of an AU-3 demapper incombination with an AU-4 mapper. In a similar way, an STM-n signalcontaining AU-4 frames can be converted into another type of STM-nsignal containing AU-3 frames.

FIG. 25 shows a second system configuration according to the presentinvention. As previously described, the VC-3 frame in AU-3 signals isdifferent from that in AU-4 signals in the arrangement of stuff columns.The second system configuration copes with this difference, employing alow-order group interface 510 which maps and demaps (or multiplexes anddemultiplexes) both AU-3 and AU-4 signals. This low-order groupinterface 510 comprises a demapper/demultiplexer section (i.e., theleft-hand blocks in FIG. 25) and a mapper/multiplexer section (theright-hand blocks). The blocks named “AU-PTR-DET” and “AU-PTR insert”perform the termination and insertion of AU pointer in the course ofdemultiplexing and multiplexing processes. For ease of subsequentcross-connect processing, a fixed value (e.g., 522) is given to the AUpointer at this stage.

Other functional blocks of the illustrated network element include: ahigher-rate interfaces 502 and 503, a cross-connect unit 504, and a CPUcontroller 520. The cross-connect unit 504 supplies the low-order groupinterface 510 with higher-level frames in an appropriate internal signalformat (e.g., STM-4/STS-12 level). The low-order group interface 510demultiplexes or demaps those incoming signals, depending on whichhierarchical series of multiplexing levels they belong to. This enablesthe stuff bytes in a VC-3 signal to be located and processed correctly.The demapping of VC-3 signals results in low-level signals whose formatis common to the AU-3 and AU-4 series networks, which are routed to themapper/multiplexer section. (Loopback paths, however, may be created ata still lower level, if required.)

The mapper/multiplexer section has to be configurable to yield a desiredhierarchical series. To this end, the low-order group interface 510employs, for example, some provision registers which can be set up froma network management console 20 through the CPU controller 520.

FIG. 26 shows a third system configuration according to the presentinvention. The illustrated low-order group interface 600 supports DS1(T1), E1 (D12), 64 Kbps, and other PDH signals, as well as signals onwhich ATM cells are mapped. Its PDH interface enables those signals tobe handled at the rate of 622 Mbps inside the low-order group interface600.

The low-order group interface 600 communicates with higher-levelfacilities (not shown) at the TUG-2 level (seven TUG-2 signals aremultiplexed into a single STS-1/TU-3/VC-3 frame). The low-order groupinterface 600 provides functions to demultiplex those TUG-2 signals downto TU-12 (VT-2) and TU-11 (VT-1.5) levels, as well as multiplexingsignals in the opposite direction. The TU-11 multiplexer (MUX) anddemultiplexer (DMUX) are linked to the VC-11 mapper and demapper,respectively. Likewise, the TU-12 multiplexer and demultiplexer arecoupled to the VC-12 mapper and demapper, respectively. Further, theTU-12 demultiplexer has a link to the VC-11 demapper, and the VC-11mapper has a link to the TU-12 multiplexer.

The above-described system converts a high-level signal containing TU-11to a TU-12 containing signal as follows. Suppose, for example, that asignal carrying DS1 (T1) traffic is to be remapped into TU-12. In thiscase, the low-order group interface 600 has to be configured in such away that the VC-11 output of the TU-11 demultiplexer will be looped backto the TU-12 multiplexer, instead of routing it to the VC-11 demapper.This enables the VC-11 virtual containers to be aligned into TU-12tributary units and then multiplexed into TUG-2 signals. (If necessary,the loopback path may be created at a still lower level.)

FIG. 27 shows a fourth system configuration according to the presentinvention. This system comprises higher-rate interfaces 702 and 704, across-connect unit 703, and lower-rate interfaces 710. One high-rateinterface 702 is connected to an “ATM over SONET” network, while theother high-rate interface 704 to an “ATM over SDH” network. Thelower-rate interface 710, on the other hand, serves for such channels as“ATM the over DS3,” “ATM the over PLCP over DS3,” and “ATM over E3.” Thefourth system configuration permits conversion between such various ATMcell-carrying frames.

The lower rate interface 710 communicates with the cross-connect unit703 at the STS or equivalent level at 622 Mbps or 155 Mbps rates. AsFIG. 27 shows, the upper part of each lower-rate interfaces 710 providesan STS demultiplexer/multiplexer. Besides demultiplexing those STS-levelsignals to extract ATM cells therefrom, the STSdemultiplexer/multiplexer maps ATM cells to the payload field of STSframes. The lower part of the lower-rate interfaces 710 comprises: anATM cell extractor, a DS3 and E3 interfaces, a DS3 demapper/mapper, anE3 demapper/mapper, and a Physical Layer Convergence Protocol (PLCP)demapper/mapper. These elements extracts ATM cells from lower-rate PDHsignals of D3 and E3, as well as mapping ATM cells into PDH frames.

In the above-described lower-rate interfaces 710, ATM cell alignment isidentified by locating each ATM cell header in the demapped frame signalstream. Now that ATM cells are extracted, some of them are looped backinside the ATM cell extractor itself. In this way, higher-level framescan be converted through the use of ATM cell loopback paths.

FIG. 28 shows a fifth system configuration according to the presentinvention. The illustrated network element comprises IP interfaces 810which interconnect various types of transport media for IP packettraffic, such as “IP over WDM,” “IP over SONET/SDH,” “IP over PPP,” “IPover ATM,” and “IP over Ethernet.” This IP interface 810 provides aninterface function that supplies a cross-connect unit (not shown) andother part of the network element with IP packet-level signals extractedfrom those signal frames. By giving appropriate destinations to such IPpacket-level signals, the IP interface 810 provides frame conversionfunctions.

Although not shown in FIG. 28, the network element may have to servemore complex transport mechanisms, such as: “IP over PPP over SONET,”“IP over ATM over SONET,” “IP over SONET over WDM,” “IP over PPP overSONET over WDM,” and “IP over ATM over SONET over WDM.” Even in suchcomplex cases, the network element can provide frame conversionfunctions by demapping those frame signals down to “IP over PPP,” “IPover ATM,” or like levels and subjecting them to the IP interface 810.

The above description of the preferred embodiments will now besummarized below. According to the present invention, the proposedtransmission unit and two-way signal conversion method producelower-level signals by converting incoming network signals down to alower hierarchical level at which the first and second network systemsare compatible with each other in terms of logical signal structure.These lower-level signals are looped back to an upward conversionprocess where given signals are multiplexed into a higher hierarchicallevel. This structural arrangement enables efficient conversions betweensignals having different specifications in terms of multiplexinghierarchy, as well as providing a better service quality. The transportsignals handled by the proposed transmission unit may be electricsignals or optical signals. In the latter case, the transmission unitmay be configured to use optical multiplexer/demultiplexer and opticalcross-connect facilities as its integral functions.

The foregoing is considered as illustrative only of the principles ofthe present invention. Further, since numerous modifications and changeswill readily occur to those skilled in the art, it is not desired tolimit the invention to the exact construction and applications shown anddescribed, and accordingly, all suitable modifications and equivalentsmay be regarded as falling within the scope of the invention in theappended claims and their equivalents.

1. A transmission unit which transmits and receives digital signals overa first and second network systems, comprising: (a) a first signalinterface means for receiving a first network signal and converting thereceived first network signal into a first internal signal with a fixedbit rate, and for sending an outgoing first network signal which isconverted from a given second remapped internal signal with the samefixed bit rate; (b) a second signal interface means for receiving asecond network signal and converting the received second network signalinto a second internal signal with the fixed bit rate, and for sendingout an outgoing second network signal which is converted from a givenfirst remapped internal signal with the same fixed bit rate; (c) atwo-way signal conversion means for making conversions between the firstand second network signal, comprising: a downward conversion meansreceiving the first and second internal signals and producinglower-level signals by converting the first and second internal signalsdown to a lower hierarchical level at which the first and second networksystems are compatible with each other in terms of logical signalstructures, an upward conversion means for producing a first or secondremapped internal signal by convening a given lower-level signal up to ahigher hierarchical level which complies with the first or secondnetwork system, and a looping back means for looping back, at the lowerhierarchical level, the lower-level signals from said downwardconversion means to said upward conversion means to cause thelower-level signals deriving from the received first and second networksignals to be converted to the first and second remapped internalsignals, respectively; (d) a switching means for providing andcontrolling circuit paths to route the first internal signal, the secondinternal signal, the first remapped internal signal, and the secondremapped internal signal; and wherein said two-way signal conversionmeans makes the conversions between the first and second network signalswhich include at least one of: two-way conversions between SynchronousDigital Hierarchy(SDH) signals and Synchronous Optical Network(SONET)signals; two-way conversions between Plesiochronous DigitalHierarchy(PDH) signals; and two-way conversions between AsynchronousTransfer Mode(ATM) signals.
 2. The transmission unit according to claim1, wherein: said downward conversion means terminates overheadinformation contained in the first and second internal signals duringthe downward conversion; and said upward conversion means insertsoverhead information to the first and second remapped internal signalsduring the upward conversion.
 3. The transmission unit according toclaim 1, wherein: said downward conversion means locates stuff datacontained in the first and second internal signals, and removes thestuff data during the downward conversion; and said upward conversionmeans inserts stuff data to the first and second remapped internalsignals, considering which part of such signals should be stuffed so asto comply with the first and second network systems, respectively. 4.The transmission unit according to claim 1, wherein said two-way signalconversion means makes transmission rate conversions between the firstand second network signals.
 5. The transmission unit according to claim4, wherein said two-way signal conversion means performs at least oneof; a conversion from Tributary Unit 11 (TU-11 to TU-12; a conversionfrom TU-11 to Virtual Tributaries 2(VT-2); a conversion from VT-1.5 toTU-12; and a conversion from VT-1.5 to VT-2.
 6. The transmission unitaccording to claim 1, wherein: said downward conversion means extractsATM cells from the first or second internal signal during the downwardconversion; and said upward conversion means produces the first orsecond remapped internal signal by inserting the ATM cells during theupward conversion.
 7. The transmission unit according to claim 1,wherein the received first and second network signals contain InternetProtocal(IP) packets; said downward conversion means produces thelower-level signals by converting the first and second internal signalsinto signals having a common format; and said upward conversion meansproduces the first and second remapped internal signals by convertingupward the common format signals.
 8. The transmission unit according toclaim 1, wherein said two-way signal conversion means makes theconversions between the first and second network signals which includeat least one of: two-way conversation between high-order group signalsbelonging to different hierarchical series of signals; two-wayconversions between low-order group signals belonging to differenthierarchical series of signals; and two-way conversions between ahigh-order group signal and a low-order group signal which belong todifferent hierarchical series of signals.
 9. The transmission unitaccording to claim 1, wherein said two-way signal conversion means makesthe conversions between the first and second network signals, based onAdministrative Unit (AU) pointer types identified.
 10. The transmissionunit according to claim 1, wherein said two-way signal conversion meansmakes the conversions between the first and second network signals,based on a value given in a byte in a frame overhead.
 11. Thetransmission unit according to claim 1, wherein further comprising meansfor interfacing with a network management console which is used inoperations and maintenance of the conversions between the first andsecond network signals.
 12. The transmission unit according to claim 1,comprising a low-order group interface which processes low-order groupsignals, wherein said two-way signal conversion means is employed as anintegral part of said low-order group interface.
 13. A transport systemwhich transmits and receives digital signals over a first and secondnetwork systems, comprising: (a) a first transmission unit comprising: afirst signal interface means for receiving a first network signal andconverting the received first network signal into a first internalsignal with a fixed bit rate, and for sending out an outgoing firstnetwork signal which is converted from a given second remapped internalsignal with the same fixed bit rate; a first downward conversion meansreceiving the first internal signal and producing a first lower-levelsignal by converting the first internal signal down to a lowerhierarchical level at which the first and second network systems arecompatible with each other in terms of logical signal structure, and afirst upward conversion means for producing a first remapped internalsignal by converting a given second lower-level signal up to a higherhierarchical level which complies with the first network system; (b) asecond transmission unit comprising: a second signal interface means forreceiving a second network signal and converting the received secondnetwork signal into a second internal signal with the fixed bit rate,and for sending out an outgoing second network signal which is convertedfrom the first remapped internal signal; a second downward conversionmeans receiving the second internal signal and producing the secondlower-level signal by converting the second internal signal down to alower hierarchical level at which the first and second network systemsare compatible with each other in terms of logical signal structure, anda second upward conversion means for producing the second remappedinternal signal by converting the first lower-level signal up to ahigher hierarchical level which complies with the second network system;(c) a looping back means for looping back, at the lower hierarchicallevel, the first and second lower-level signals from said first andsecond downward conversion means to said second and first upwardconversion means respectively, to cause the lower-level signals derivingfrom the received first and second network signals to be converted tothe first and second remapped internal signals, respectively; andwherein the conversions between the first and second network signalsinclude at least one of: two-way conversions between Synchronous DigitalHierarchy(SDH) signals and Synchronous Optical Network(SONET) signals;two-way conversions between Plesiochronous Digital Hierarchy(PDH)signals; and two-way conversions between Asynchronous Transfer Mode(ATM)signals.
 14. A two-way signal conversion method which converts networksignals between a first and second network systems, comprising the stepsof: (a) receiving a first network signal and converting the receivedfirst network signal into a first internal signal with a fixed bit rate,(b) receiving a second network signal converting the received secondnetwork signal into a second internal signal with the fixed bit rate,(c) producing lower-level signals by converting the first and secondinternal signals down to a lower hierarchical level at which the firstand second network system are compatible with each other in terms oflogical signal structure; (d) producing higher-level signals byconverting each given lower-level signal up to a higher hierarchicallevel which complies with the first or second network system; (e)looping back the lower-level signals produce at said step (d), wherebythe lower-level signal resulting from the first internal network signalwill be converted into an outgoing signal to the second network system,and the lower-level signal resulting from the second internal networksignal will be converted into an outgoing signal to the first networksystem wherein the conversions of the network signals include at leastone of: two-way conversions between Synchronous Digital Hierarchy(SDH)signals and Synchronous Optical Network(SONET) signals; two-wayconversions between Plesiochronous Digital Hierarchy(PDH) signals; andtwo-way conversions between Asynchronous Transfer Mode(ATM) signals. 15.The two-way signal conversion method according to claim 14, wherein:said step (c) of producing the lower-level signals comprises terminatingoverhead information contained in the received first and second networksignals during the downward conversion; and said step (d) of producingthe higher-level signals comprises inserting overhead information to theoutgoing signals during the upward conversion.
 16. The two-way signalconversion method according to claim 14, wherein: said step (c) ofproducing the lower-level signals comprises locating stuff datacontained in the received first and second network signals, and removingthe stuff data during the downward conversion; and said step (d) ofproducing the higher-level signals comprises inserting stuff data to theoutgoing signals, considering which part of such signals should bestuffed so as to comply with the first and second network system. 17.The two-way signal conversion method according to claim 14, wherein saidstep (d) of producing the higher-level signals comprises convertingtransmission rates of the received first and second network signals. 18.The two-way signal conversion method according to claim 17, wherein theconversion of transmission rates include at least one of: a conversionfrom Tributary Unit 11 (TU-11 to TU-12; a conversion from TU-11 toVirtual Tributaries 2(VT-2); a conversion from VT-1.5 to TU-12; and aconversion from VT-1.5 to VT-2.
 19. The two-way signal conversion methodaccording to claim 14, wherein: said step (c) of producing thelower-level signals comprises extracting ATM cells from the receivedfirst or second network signal during the downward conversion; and saidstep (d) of producing the higher-level signals comprises inserting theATM cells to the outgoing signals during the upward conversion.
 20. Thetwo-way signal conversion method according to claim 14, wherein: thereceived first and second network signals contain Internet Protocal(IP)packets; said step (c) of producing the lower-level signals comprisesconverting the first and second incoming network signals into signalshaving a common format; and said step (d) of producing the higher-levelsignals comprises converting upward the common format signals.
 21. Thetwo-way signal conversion method according to claim 14, wherein theconversion of the network signals including at least one of: two-wayconversion between high-order group signals belonging to differenthierarchical series of signals; two-way conversion between low-ordergroup signals belonging to different hierarchical series of signals; andtwo-way conversion between a high-order group signal and a low-ordergroup signal which belong to different hierarchical series of signals.22. The two-way signal conversion method according to claim 14, whereinsaid step (c) of producing the lower-level signals comprises:identifying Administrative Unit (AU) pointer types of the first andsecond internal network signals; converting the first and secondinternal network signals, based on the identified AU pointer types. 23.The two-way signal conversion method according to claim 14, wherein saidstep (c) of producing the lower-level signals comprises: identifying avalue given in a byte in a frame overhead of each first or secondinternal network signal; and converting the first and second internalnetwork signals, based on the identified byte values.
 24. The two-waysignal conversion method according to claim 14, further comprising thestep of using a network management console for operations andmaintenance of the conversion of the network signals.